Microphones used in automotive electronic applications, such as cell phones, navigational systems, and vehicular control, are well-known in the art. An automotive vehicle presents challenges to the use of a microphone in view of the numerous sources of noise that can interfere with vocalized speech inside the vehicle. These challenges can be particularly difficult when adapting a microphone solution for use in the vehicular rearview mirror assembly. In addition to the difficulties of rejecting noise within the vehicle, disturbances to the sound field caused by the rearview mirror, windshield and other surfaces must also be addressed.
The prior art includes systems that use microphones positioned in tandem, i.e., a first microphone positioned in front of a second microphone. This type of system works to produce a difference signal for canceling noise by subtracting the signals and using a delay to account for the distance between the microphones. However, the rearview mirror disturbs the sound field between the two microphones, which results in poor subtraction over much of the frequency range of interest. Additionally, this front and back microphone configuration requires the rearview mirror to include a deeper housing for supporting the rearward microphone, which is often an undesired design feature in view of styling, weight, vibration sensitivity, and molding required in the manufacturing process. It may be desirable to use silicon microphones based on MEMS technology due to their long term stability, small size and ease of use in a mass production environment.
Other prior art systems have used microphones that were positioned in parallel that use digital processing or simple delay networks to improve operation. The use of digital processing introduces delay and variation over time that disrupts systems designed for a single microphone. Therefore, this type of simple delay based processing does not yield the desired performance. Additionally, many of the microphone systems currently in use were developed under the assumption that the microphone would be used in connection with a handheld mobile phone. In handheld applications, the very close proximity of the user's mouth to the microphone assures a very high speech-to-noise content for most situations. These systems do not function correctly with microphones used at a distance because audio received at increased distances does not exhibit the same frequency characteristics.
Microphones distant from an audio source that are used in a hands-free automotive systems will often have a very significant noise content, and manifest a wider dynamic range. A “close use” situation or microphone may be defined as one positioned within 20 cm of the audio source such as a user's mouth. The dynamic range is increased because of the broader range of possible speech signal levels and relative noise content. In a distant use situation, if a wider dynamic range speech signal is processed via the phone system, especially phones employing code division multiple access (CDMA), much of the desired speech can be lost because the processing system (CODEC) is unable to correctly determine that speech is present. Thus, the phone system functions as if a voice plus noise signal is comprised of only noise.
Many noise reduction systems as used in the prior art seek to lower only the noise content while retaining the speech in its unaltered state. This process does not restore the nature of the speech signal to that of a close use microphone as found in a typical handset and as a result does not yield a signal able to pass through the cell phone's CODEC. Also, most single channel noise reduction algorithms reduce noise but do not necessarily improve speech intelligibility for humans or machines. The only algorithms consistently shown to improve intelligibility are based on directional processing and necessarily utilize two or more microphones. With typical processing, there will be many frequency bands or occurrences where the speech content, though significant, is not great enough to overcome the residual noise to the extent so as to avoid being interpreted as noise. Thus, in latter processing stages, these frequency bands or occurrences will be removed because they appear to be only unwanted noise. Even though the speech content is significant, it is not of a great enough magnitude to overcome the noise in certain frequency bands or at certain times.
Many different types of noise reduction systems are known in the art for reducing internal noise within a vehicle. Some of these systems operate to reduce internal vehicular noise using digital signal processing techniques. Digital signal processing (DSP) refers to the representation of discrete time signals by a sequence of numbers or symbols that are subsequently processed. The goal of DSP is usually to measure, filter and/or compress continuous real-world analog signals. DSP algorithms have long been run on standard computers, programmable gate arrays, on specialized processors called digital signal processors, or on purpose-built hardware such as ASICs.
One such DSP technique used to mitigate noise involves the use of least mean squares (LMS) algorithm. LMS algorithms are a class of adaptive filters used to mimic a desired filter by finding the filter coefficients that relate to producing the least mean squares of the error signal i.e. the difference between the desired and the actual signal. The LMS algorithm operates using a stochastic gradient descent method since the filter is only adapted based on the error at the current time. This type of DSP technique typically operates by obtaining a filter coefficient by approximating the gradient in order to simplify the calculation and then utilizing it in an adaptive filter correction formula such as: LMS, normalized least mean squares (NLMS), Affine Projection, proportionate normalized least means square (PNLMS) or other adaptive algorithm. Although LMS algorithms have been practically used in vehicle applications, these techniques often use a prerecorded reference signal which is not robust against changes in vehicle acoustics or driver position.
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